My understanding is that most hose can, and does stretch and elongate under pressure, thus if a sudden break occurs, the result can be a rapid return to normal length, resulting in the hose moving dangerously fast. During testing the pressure is is contained and higher thus more elongation (easily seen in testing). Requiring a shorter length minimizes the amount of "snap-back" that can occur. It also is a good reason to keep lengths straight as possible as that limits the direction of the rapidly shrinking hose?

I have also heard from some that the woven jackets may not be perfectly straight and parallel with the liner, contributing to less predictable movement upon a burst?

I am interpreting your question as why the standard friction loss testing requires 100 feet of inlet hose, 100 feet of hose that is being tested, and 100 feet of out let hose. You are asking why can't just the one hose being tested be used?

Friction loss in the hose comes from the turbulence of the flowing water, think of an eddy (Eddy (fluid dynamics) - Wikipedia) any roughness on the hose lining causes mini eddy in away meaning the water near the lining is flowing slower than the water in the center. This is also why smaller ID hoses have higher friction loss than Large Diameter Hoses, because the proportion of water near the lining compared to "free" flowing in the center is much less and does not exert as much turbulence, or the total volume of water has much less resistance to flow (friction).

The same concept is true when waterway changes diameter, from bigger to smaller water way will increase pressure resistance, and the opposite is so from small to large. In either case the change will cause eddy's and disrupt the flow. To reduce this you want to have the water conditioned to a flow that is the same as the test hose before it enters and flow into the same environment after the test hose. If this is not done than the friction loss you record will have other factors affecting it rather than just the hose you are testing.

If your fire department uses 50 foot sections rather than 100 it may be wise to use two 50 foot hoses for your one hundred feet as each coupling the water travels will increase the friction loss, you will want to take this into account have usable numbers when on the fire scene.

I could be wrong, but I think the question is why can't FD's conducting a service test on hose lay than 300 feet from any single discharge? This is detailed in NFPA 1962, 4.8.4.4 - The total length of any hose line in the hose test layout to be service tested shall not exceed 300 feet.

And low and behold, when I click the asterisk for the annex info I found what I'd been shown years ago. From NFPA 1962, 2013 Ed:

"A.4.8.4.4 Hose is tested in lengths not exceeding 300 ft (91 m) to allow the hose to untwist and be straightened out. As the pressure rises, the shorter length will allow the hose to assume a natural elongation, creating less warp in the hose.It is also important that all the air in the hose be removed. If any point in the hose layout is elevated, air will be trapped at that point. Excessive lengths make it difficult to exhaust all the air. The ideal hose test area will have a slight upward incline from the pressure source to the capped end. This allows the air to flow to the capped end and be bled off. There should be no humps or valleys in the hose between the ends, as these will trap air".

I think you may be describing best practices for determining friction loss in a given 100 feet of hose?